A long-recognized important objective in the constant advancement of digital IC (Integrated Circuit) technology is faster speed and lower power dissipation. For example, a digital system such as a modern microprocessor is comprised of MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) operating at a low rail-to-rail voltage of 1.5 V. In addition, at such a low operating voltage, the gate oxide is designed to be in the range of tens of angstroms (.ANG.). Such a thin gate oxide is advantageous for smaller device geometry and for faster switching speed of the MOSFETs.
Furthermore, the digital system typically must interface with other electronic systems which operate at higher voltages. For example, typical communications systems operate at a rail-to-rail voltage of 3.3 V. Thus, when the digital system having the lower core voltage range of 1.5 V interfaces with the external system having the higher external voltage range of 3.3 V, a level shifter is used within the digital system to shift the lower core voltage range to the higher external voltage range.
However, the thin gate oxide of MOSFETs in the level shifter of the digital system may be adversely affected by the higher external voltage range of an external system. The gate oxide may break down if the gate is biased to a high operating voltage or the operating life time of a MOSFET having the thin gate oxide may be diminished if the gate is biased to a high operating voltage.
Nevertheless, a level shifter, within a digital system having MOSFETs with thin gate oxides, is needed to shift the lower core voltage range to the higher external voltage range when the digital system interfaces to the external system. However, since the level shifter is comprised of MOSFETs having thin gate oxide, a mechanism is desired for protecting the MOSFETs in the level shifter while the level shifter shifts the low core voltage range of the digital system to the higher external voltage range of the external system.